1. Field of the Invention
The present invention relates to an organic electroluminescent element (light-emitting element, or EL element) that converts electric energy into light and emits the light.
2. Description of the Related Art
Numerous programs in research and development on various display elements are underway today, and among them, organic electroluminescent (EL) elements, which emit high-brightness light at low voltage, have been attracting attention as promising display elements.
However, organic electroluminescent elements remain significantly lower in luminous efficiency than inorganic LED elements and fluorescent lamps.
According to Thompson et al., external energy efficiency, an indicator of the luminous efficiency of an organic EL element, can be expressed as the product of internal energy efficiency and light output efficiency of an element (“Optics Letters”, 1997, Vol. 22, No. 6, p. 396). That is, for improvement of the luminous efficiency of an organic EL element, it is necessary to improve the internal energy efficiency as well as the light output efficiency of the element.
Light output efficiency is a ratio of the light emitted from the front transparent substrate face of an element into the air to the light generated in the element. The light generated in the light-emitting layer passes through a number of interfaces between media different in refractive index before it is emitted into the air. According to Snell's Law of Refraction, light entering into each interface at an angle of not less than its critical angle is reflected totally by the interface back to the original layer and consequently is eliminated or released from the side face of the layer, resulting in a decrease in the amount of the light emitted from the front face of the element. As a result, for example, when the element is applied to a display, the display has lower front face brightness.
To overcome such problems, methods of suppressing the total reflection of light, using a prism structure effective in converging light have been proposed (see, for example, Japanese Patent No. 2931211).
This method involves preparing a substrate by placing a prism sheet manufactured by 3M on a glass having a thickness of 0.3 mm and adhering another glass having a thickness of 0.3 mm over the prism sheet, and then forming the organic EL element on the first glass to which the prism sheet is connected. The prism sheet, which is effective in converging light, alleviates the decrease in light output efficiency due to total reflection.
Also disclosed is a method of coating a thermosetting resin on a glass substrate to a thickness of 3 μm, hardening the resin by heating under an applied mold having prisms of 2 μm in base length and 2 μm in height, coating and hardening a polyimide resin thereon after removal of the mold and smoothing the surface of the resin film, and forming an organic EL element thereon (see, for example, JP-A No. 2003-86353).
The prism structure described in the method above is mainly made of an organic polymer material. Generally, organic polymers have a low refractive index, normally of around 1.35 to 1.5. In contrast, when a low-molecular weight material is used for the light-emitting layer of an organic EL element, the refractive index thereof is normally 1.7 or more at around the emission light wavelength, and in a phosphorescence-emitting element, is generally still higher (1.8 or more).
For prevention of total reflection of the light emitted from the light-emitting layer having a high-refractive index at the interface with a low-refractive index layer, it is necessary to make the refractive index of all the layers present between the light-emitting layer and the prism structure not less than the refractive index of the light-emitting layer, and the refractive index of the prism structure not less than the refractive index of the light-emitting layer.
It is usually possible to raise the refractive index of the prism structure by using a transparent inorganic material instead of the organic material and to improve the heat resistance thereof. Use of an inorganic material also eases restrictions in producing the element and improves the stability of the element during storage.
However, it is generally difficult to micro-fabricate a transparent inorganic material such as glass, and thus there are no known prism structures made of a transparent inorganic material.
In addition, there are no known organic EL elements having a prism structure made of such a high-refractive index material.